HUP0900569A2 - Method and automatic instrument for reducing alkali metal content of water by voltage transforming electrosorption - Google Patents

Description

PUBLICATION COPY

Service invention

Method and automatic apparatus for reducing the alkali metal ion content of water by voltage-switched electrosorption

Tisza Chemical Combine Plc., Tiszaújváros

Inventors: István Lörincz 25%, János Bóta 15%, Dr. Gábor Nagy 10%, Dr. Géza Horváth 25%, Dr. Imre Tóth 15%, Dóra Pethő Rippelné 10%.

The subject of the invention is a method and an automatic device for carrying it out for reducing the alkali metal ion content of water by voltage-switched electrosorption, especially for the purification of alkaline wastewater.

According to the current state of the art, methods based on ion exchange or calcination are used to reduce the Na ion concentration in wastewater.

The disadvantage of the ion exchange process is that the exhausted ion exchange material must be regenerated. During regeneration, more than twice the original salt content is produced in this method. This must be utilized and disposed of properly due to environmental regulations.

Reducing the sodium content can also be achieved by calcination, when NaHCO ₃ crystals are separated by a chemical reaction and calcined soda is produced as a result of heat treatment. This method requires a lot of heat and has the disadvantage that the sodium content of the caustic soda used in the technology appears as soda as a product and thus cannot be traced back to the technological process.

A theoretical electroadsorption method for the purification of NaCl-containing solutions was described by Oren and Softer (Y. Oren, A. Soffer; J. Electrochem. Soc., 125 (1978), 869-875) using a column apparatus, with the separation of ions achieved by cyclic flow of the liquid. In this method, the electrodes must be separated from each other by a membrane.

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Knowing the composition of the wastewater, our goal is to implement a technology that, in addition to reducing the Na-ion content of the wastewater, also enables the enrichment of the Na-ion pollutant in another system and its return to the technology. As a result, the emission of harmful substances into the environment can be significantly reduced.

To achieve our goal, we found that among the many alternative purification technologies, Electro Swing Adsorption (ESA) is the most suitable for reducing the alkali metal ion content. We realized that we can achieve ion enrichment without the use of a membrane if we use two separate liquid compartments and, after the electroadsorption step, we tear an electrode rich in ions on its surface out of the solution and transfer it to another solution.

ESA is based on electrosorption processes, where electrosorption refers to adsorption on the surface of electrically charged electrodes.

The absolute advantage of using electrochemical methods, including electrosorption, is that no chemicals are needed, thus no environmental pollutants are produced. Another advantage is that the electrochemical parameters can be easily measured and controlled, so the operation can be automated.

The application of voltage-switched adsorption as a separation is made possible by the fact that in an electric field the concentration of ions on the surface of the electrodes differs from that in the solution. Thus, the concentration of cations on the cathode surface increases, while the concentration of cations on the anode decreases.

The schematic structure of voltage-converter adsorption is shown in Figure 1. Where:

1. voltage source,

2. polarity switch,

3. counter electrode,

4. working electrode,

5. a vessel containing the solution to be depleted,

6. a vessel containing the solution to be enriched.

According to our invention, voltage-switched adsorption is applied in a cyclic operation by first applying a negative potential to the electrode, and then, when the cations have become enriched on its surface, the working electrode is removed from the solution and immersed in another solution, where the polarity is reversed. As a result, the cathode with a cation-rich surface becomes the anode, from which the cations diffuse into the solution under the influence of the electric field. The detachment of the cations |TVKNyrt. 2 Cg.: 05-10-000065| *: S2oígálati’tiíálfr£irJ

--------------—--“ · · · · · · · « · · After that, the working electrode is lifted back into the first solution and the process is started again. By using an appropriate cycle, the cation, in this case the Na-ion concentration, in the solution to be depleted is reduced to the desired level, while the Na-ion concentration of the solution to be enriched is high enough to be returned to the technology.

The expectation for an automated device is that the parameters can be adjusted quickly and in a wide range, the operation should be reliable, stable, easy to handle, and it should be able to measure flowing systems.

The embodiment of the automatic device according to the invention and the associated service equipment is described with the help of Figure 2. Where:

1. process control computer,

2. power supply,

3. control unit,

3.1 adjustable timers,

3.2 control lamps,

3.3 programmable cycle counter,

4. the automatic adsorption device.

The basic control is performed by the process control computer 1, on which the polarization voltages can be set, and it also performs the sampling. The progress of electrosorption - desorption is indicated by changes in the current strength, therefore the computer continuously measures and registers the current strength flowing through the cells. A power supply 2 is connected to the computer, which generates the necessary voltages. The control unit 3 is located between the adsorption device and the voltage source, with two adjustable time switches 3.1 of which the selected adsorption and desorption times can be precisely set. The current status of the adsorption and desorption processes is indicated by the control lamps 3.2. The desired cycle number can be set on the programmable cycle counter 3.3. The main element of the system is the automatic adsorption device 4.

The structure and elements of the 4 automatic adsorption devices are shown in Figure 3. Where:

1. anode space tank,

2. cathode chamber tank,

3. graphite counter electrodes, iTVKNyrt.

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4. movable working electrodes,

5. transfer lever,

6. driven axle,

7. program dial,

8. electric motor,

9. microswitches.

The movable working electrodes 4 are fixed to a driven shaft 6, which is lifted from the anode compartment tank 1 to the cathode compartment tank 2 by a lifting arm 5. The lifting arm 5 is moved by the driven shaft 6, at one end of which there is a program dial 7 and four microswitches 9 that fit it. These microswitches 9 are responsible for stopping the lifting arm 5 in a given position, switching the polarization voltages on and off and changing the poles. An electric motor 8 is connected to the other end of the driven shaft 6 through a gear, which rotates the driven shaft 6. Graphite counter electrodes 3 are fixed in the tanks 1-2. The movable working electrodes 4 are inserted between these.

The operation of the automatic adsorption device is explained by presenting the following specific example:

A space suitable for holding 50 cm ³ of liquid was created in anode compartment 1 and cathode compartment 2, in which 6 graphite counter electrodes are fixedly placed.

We used 5 Ni electrodes with high porosity as the working electrodes, which are immersed in the solution on the anode side between the graphite counter electrodes. The appropriate polarization voltage is applied to the cell thus assembled so that the working electrode is the negative (cathode) and the counter electrode is the positive (anode). Then the positively charged ions migrate to the working electrodes and bind to their surface. This is the electroadsorption phase. The saturation of the working electrode can be monitored by measuring the current flowing through the cell. When the electrosorption process has taken place, the working electrode saturated with ions is removed from the solution and the polarization voltage is switched off. Then we transfer it to the other solution (cathode space) that we want to concentrate, and then we apply an opposite voltage to it. Then the working electrode becomes the positive, while the counter electrode in the other cell is the negative. As a result, the ions are desorbed from the surface of the working electrode due to electrostatic repulsion and are transferred to the solution to be concentrated. This process can also be monitored by monitoring the current flowing in the cell. After this, the working electrode is removed again, the voltage is switched off, and the cycle can start again. It is important that the electrode |TVK Open

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After removing the service electrode, turn off the polarization voltage, because a potential difference may develop between the working electrode and the solution at rest or as a result of movement.

The basic goal is to remove as many Naions as possible from the anode solution and transfer them to the cathode solution with minimal time and energy consumption.

We investigated the effect of each parameter on mass transfer. Mass transfer, under the given parameters, is the amount of ions transferred from the anode space to the cathode space per m ² during a single cycle. (Unit: mass of transferred ions / m ² , number of cycles) During the tests, we varied the cycle number, polarization voltages, solution concentration and desorption time in order to achieve the appropriate mass transfer and efficiency. The measurements were performed with NaOH solutions of 0.2 m/m %, 2 m/m %, 4 m/m % concentration and the effect of concentration on mass transfer was investigated as a function of the cycle number. During the experiments, the following parameters were kept constant:

• Polarization voltage: 1600mV • Adsorption time (ta): 25 seconds • Desorption time (td): 120 seconds • Cycle number: 80 pcs • Electrode distance: 2.5mm

Table 1

Dependence of mass transfer on concentration and cycle number

cycle number (pcs) concentration (m/m %) the (s) td (s) transfer (mg Na/m ² ) 10 0.2 25 120 326 2 25 120 7584 4 25 120 10051 20 0.2 25 120 774 2 25 120 7787 4 25 120 11255 40 0.2 25 120 1281 2 25 120 10103 4 25 120 18110 80 0.2 25 120 2968 2 25 120 19439 4 25 120 33210

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The effect of polarization voltage on mass transfer is illustrated by the following experimental data. Here, the polarization voltage was varied between 1000 mV and 2400 mV in 200 mV steps. All measurements were performed with the following constant parameters:

• Solution concentration 4% m/m • Adsorption time (ta): 25 seconds • Desorption time (td): 120 seconds • Cycle number: 80 pcs • Electrode distance: 2.5mm

At 1400-1600 mV, there is a minimum of mass transfer, since at this polarization potential water splitting begins, which inhibits the processes taking place on the surfaces. At higher voltages, this inhibition is somewhat offset by the larger electrostatic field, so mass transfer increases.

Table 2

Dependence of mass transfer on polarization voltage

U(mV) the (s) td (s) transfer (mg Na/(m ² cycle)) 1200 25 120 241.04 1400 25 120 151.84 1600 25 120 145.04 1800 25 120 371.79 2000 25 120 489.00 2200 25 120 557.72 2400 25 120 ________422.69________

In the following, we present the effect of desorption time on mass transfer. When using porous electrodes, we have to take into account the pore diffusion inhibition that occurs. We have found that adsorption processes occur faster than desorption. As a result, the electrode becomes saturated with sodium ions after a certain time.

In order to increase the mass transfer, we also increased the desorption time. The adsorption time was 25 s, while the desorption time was varied between 25 - 600 s. Based on the experiments, it can be said that due to the pore diffusion inhibition, we should use a desorption time of at least 300 s.

|TVK Open.

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During the tests, we measured the mass transfer rate as a function of desorption time under the following constant parameters:

• Solution concentration • Polarization voltage • Adsorption time (ta):

• Cycle number:

%m/m

1200 mVs

pcs • Electrode distance:

2.5mm

Table 3

Dependence of mass transfer on polarization voltage

the (s) td (s) transfer (mg Na/(m ² cycle)) 25 60 359.2 25 90 401 25 120 426.8 25 210 470.5 25 240 637.3 25 300 1171 25 600 1234

Of course, the processes taking place depend on the pore size distribution and structure of the electrode. The determined desorption time (300 s) applies only to Ni-pellet electrodes prepared according to the same recipe. For other types of electrodes, this value must be determined again, since the pore size distribution also depends to a large extent on the electrode manufacturing conditions.

The advantage of using voltage-switched electrosorption is that it does not use hazardous chemicals, only inert electrodes. Furthermore, it does not produce any environmental pollutants, and the electrochemical parameters are simple to measure and easy to control.

The process can be used for the treatment of olefin plant and refinery wastewaters, and for the desalination of waters, provided that their alkali ion concentration is between 0.2-4 m/m %. We have found that the system can operate with a purification efficiency of 20-30 %, which can be improved if operated in a cascade system.

|TVK Plc.

Ref.: 05-10-000065|

Claims (3)

SMgálatrtáláJfnSntf Requirements 1. A method for reducing the alkali metal ion content of water by voltage-switched electrosorption, characterized in that voltage-switched adsorption is applied in a cyclic operation by first applying a negative potential to the electrode, then when the cations have become enriched on its surface, the working electrode is lifted out of the solution and immersed in another solution, where the polarity is reversed, after the cations have been detached, the working electrode is lifted back into the first solution and the process is repeated. 2. The method according to claim 1, characterized in that porous Ni working electrodes connected to a negative polarity of a DC voltage of 1000-2400 mV are immersed in a cell equipped with graphite counter electrodes, in a NaOH solution with a concentration of 0.2-4 m/m %, for a period of 1-25 s, then the working electrodes are removed from the solution and immersed in another cell equipped with graphite counter electrodes, in a NaOH solution with a concentration of 0.2-4 m/m %, the polarity of the working electrodes is changed and kept in the solution for a period of 25-600 s, the working electrodes are removed from this solution and returned to the previous solution and the polarity is changed again, the cyclic operation is performed 1-80 times. 3. An automatic device for carrying out the method according to claim 1, characterized in that its 4 movable working electrodes are fixed to a driven shaft 6, which is lifted by a lifting arm 5 from the anode compartment tank 1 to the cathode compartment tank 2, the lifting arm 5 is moved by the driven shaft 6, at one end of which there is a program dial 7 and four microswitches 9 matching thereto, an electric motor 8 is connected to the other end of the driven shaft 6 through a gear, which rotates the driven shaft 6, graphite counter electrodes 3 are fixed in the tanks 1-2, between which the 4 movable working electrodes are located. Tiszaújváros, September 7, 2009. Head of Technology Development iTVK Nyrt. Ref.: 05-10-000065|